Interferon-.beta.2, now designated Interleukin-6 (hereinafter IFN-.beta.2/IL-6), is a multifunctional cytokine that regulates the growth and differentiation of various cells and tissues and appears to be one of the important mediators of the response to viral and bacterial infections and to shock. The biological effects now associated with IFN-.beta.2/IL-6 include: stimulation of immunoglobulin secretion by mature B lymphocytes (BSF-2 activity), growth stimulation of plasmacytomas and hybridomas (HGF activity), activation of T cells, stimulation of hepatic acute phase protein synthesis (HSF activity), stimulation of hematopoiesis, cell differentiation (DIF activity), inhibition of tumor cell growth (AP activity) and other IFN-like effects. As a typical cytokine, IFN-.beta.2/IL-6 is secreted by many cell types and acts in various combinations with other interleukins and interferons. Among its activities viewed as having antitumor potential are: inhibition of cancer cell growth and colony formation, differentiation of malignant cells to more normal phenotypes, stimulation of normal hematopoiesis, stimulation of T-lymphocyte activation, stimulation of antibody secretion by B-cells, stimulation of complement synthesis, and stimulation of antiprotease synthesis.
The cloning of the human IFN-.beta.2/IL-6 receptor was reported (Yamasaki, et al., Science, Vol. 241, pp. 825-828). However, the natural human IFN-.beta.2/IL-6 receptor and a soluble extracellular fragment thereof have not been isolated and described in the literature.